Process for electrolytically manufacturing aluminum



RATIO OF RADIOACTIVITY OF 46A! TO THAT OF 5F([45N]/[49F]) HSAQ FUJI! ET AL, 3,536,595

PROCESS FOR ELECTROLYTICALLY MANUFACTURING ALUMINUM Filed May 29, 1967 0'. d4 WEIGHT mm 0F OXYGEN TO FLUORINE Japan Filed May 29, 1967, Ser. No. 642,120 Int. Cl. C22d 3/12; GOlt 1/16 US. Cl. 20467 1 Claim ABSTRACT OF THE DISCLOSURE An electrolytic manufacturing process for aluminum which comprises taking samples from an operating electrolytic bath consisting of mixed molten salts of alumina and cryolite, bombarding these samples with neutrons for activation, measuring the respective radioactive intensities of N and F produced in the sample by said activation, determining the alumina content of the sample from the ratio of the radioactive intensity of N to that of F to ascertain reductions in the alumina content of the electrolytic bath and thereby carrying out electrolysis continuously under the conditions where the alumina content of the electrolytic bath can be easily maintained within the prescribed range by adjusting the additional supply of alumina to make up for reductions in its content.

BACKGROUND OF THE INVENTION This invention relates to an electrolytic manufacturing process for aluminum, and more particularly to a process for electrolytically manufacturing aluminum by activating the samples of an electrolytic bath taken from an electrolytic cell to determine the oxygen content and controlling the alumina content of the electrolytic bath to within the prescribed range on the basis of the alumina content as determined by said oxygen content.

As is well known, the electrolytic manufacturing process for aluminum is carried out by dissolving alumina in an electrolytic bath mainly consisting of molten cryolite and electrolyzing molten salts consisting of mixtures of these alumina and cryolite (Al O +3NaF-AlF In this case, however, the alumina content of the electrolytic bath gradually decreases with the progress of electrolysis, so that where the alumina content falls to below a certain level, it is necessary to increase the aluminum concentration in the electrolytic bath by makeups so as to continue electrolysis.

In electrolytic manufacturing process for aluminum, the alumina content of the electrolytic bath and the current efliciency have interrelations, namely, the higher the alumina concentration, the lower the temperature of the electrolytic bath and the more improved the current efficiency. However, the excess presense of alumina in an electrolytic cell will result in the precipitation of undissolved alumina and the decrease of efliciency. Therefore for eflicient electrolysis, it is preferable to maintain the alumina concentration in the electrolytic bath at a relatively high level and also within as constant a range as possible. While it has been desired for this purpose to devise a method for carrying out a quick and accurate determination of the alumina content of the electrolytic bath conveniently during the electrolytic operation, no such method has yet been developed. Therefore common practice in the aforesaid type of aluminum manufacturing is additionally to supply alumina to an electrolytic bath as though fit merely from experience in the electrolytic operation, for example, to carry out such make-up supnited States Patent 3,536,595 Patented Oct. 27, 1970 plies using the so-called anode effect as a guide which results from a sharp rise in the anode potential when the alumina content of the electrolytic bath has fallen to below a certain level (usually where said alumina content has been reduced to less than 1.5 percent).

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWING The attached drawing is a property curve diagram showing relations of the ratio of the oxygen content to the fluorine content of an electrolytic bath for aluminum manufacture, before samples of said bath are activated by neutron bombardment according to the process of the present invention versus the ratio of the radioactive intensity of N to that of F.

DETAILED DESCRIPTION OF THE INVENTION The process of the present invention primarily consists in taking samples from an alumina electrolytic bath in operation, bombarding these samples with neutrons so as to convert the oxygen in the sample to N by the (n, p) reaction and the fluorine thereof to F and N respectively by the 11, Zn) and (n, a) reactions ascertaining the alumina concentration in the electrolytic bath by determining the ratio of the radioactive intensity of N to that of F, and thereby adjusting the additional supply of alumina on the basis of the values of said concentration thus obtained in order to maintain it within the prescribed range.

Further description will hereinafter be given of the process of the present invention. First, samples are taken from an alumina electrolytic bath in operation and then bombarded with neutrons. The oxygen in the sample is converted to N (half life 7.35 seconds) by the (n, p) reaction caused by said neutron bombardment and at the same time a large quantity of fluorine in the sample to F (half life 112 minutes) and N respectively by the (11, Zn) and (n, a) reactions. However, since N is converted from both 0 and F as described above, it is impossible to ascertain the A1 0 content of the sample directly from the value of the oxygen content determined from the radioactive intensity of N alone. Therefore the process of the present invention has eliminated the effect of the N converted from fluorine by determining the respective radioactive intensities of N and F converted by neutron bombardment. Namely, the process of the present invention consists in measuring the radioactive intensities N] and F] of N and F respectively immediately after neutron bombardment to obtain the ratio of [*N] to F] thereby ascertaining the oxygen content of the sample, computing the alumina content corresponding to said oxygen content and consequently reductions in the alumina content of the electrolytic bath and thereby carrying out electrolysis continuously under the conditions where the alumina content of the electrolytic bath can be easily maintained within the prescribed range by adjusting the additional supply of alumina to make up for reductions in its content.

The process of the present invention will be understood more clearly with reference to the example which follows.

It should be understood, however, that they are oifered only by way of illustration and are not intended to restrict the scope and breadth of the present invention or limit the scope of the claim.

EXAMPLE Cryolite-base samples containing 0, 3, 5 and 10 percent by weight of alumina respectively were prepared and bombarded with neutrons of 14 mev. under the fixed conditions to activate the oxygen and fluorine of each sample. Upon completion of (namely, 2 or 3 seconds after) the bombardment N] was first determined in 30 seconds and, more than 10 minutes later, [3+ (positrons) emitted from F was measured by a 'y'y coincidence system to determine the intensity F]. This determination was further supplemented by correcting the measured values so as to allow for the aforesaid lapse of more than 10 minutes, the results obtained being shown in the table below. Correction for this time lapse was carried out by dividing the values by 2' (where A is a decay constant and t is time).

TABLE Weight ratio of oxygen to fluorine Ratio of ["*N] to Alumina content, percent (O/F) ](l l) of said oxygen content. Hence, it is possible to continue electrolysis under the conditions where the alumina concentration in the electrolytic bath can be easily kept within the prescribed range by adjusting the additional supply of alumina to make up for reductions in its content. Incidentally, the time required in all these determinations is only about minutes.

What is claimed is:

1. A process for electrolytically manufacturing aluminum which comprises sampling part of an operating electrolytic bath consisting of mixed molten salts of alumina and cryolite; bombarding the bath samples with neutrons for activation; measuring the radioactive intensities of N and F converted by the aforesaid activation, respectively from the oxygen and fluorine in the bath samples; determining the oxygen content of the bath samples from the ratio of the radioactive intensity of N to that of F to ascertain the alumina content corresponding to said oxygen content and consequently reductions in the alumina content of the electrolytic bath and thereby carrying out electrolysis continuously under the condition where the alumina content of an operating electrolytic bath can be easily maintained within the prescribed range by adjusting the additional supply of alumina to make up for reductions in its content.

References Cited UNITED STATES PATENTS 3,244,604 4/1966 Diller 20467 3,392,092 7/1968 Diller 20467 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 25083.3, 106 

